(Reuters) -Delta Air Lines struggled on Sunday to restore normal operations after last week's crippling global cyber outage, canceling 1,250 flights beyond the 3,500 it had already scrapped.

The issue has stranded thousands of Delta travelers across the United States, with some having to rent cars to drive hundreds of miles while others could have to wait days for new flights or cancel trips altogether.

The Atlanta-based airline, which canceled a third of its schedule and delayed another 1,700 flights or 44%, according to FlightAware, is battling operational issues after the outage hit its crew tracking system.

Delta has offered no timetable for resumption of normal operations and has already canceled another 305 flights for Monday, the flight tracking website said. Its total of canceled flights since Friday now stands at more than 5,000.

A software update by global cybersecurity firm CrowdStrike triggered system problems for Microsoft customers, including many airlines, on Friday.

Although other U.S. airlines have largely recovered, Delta has struggled to return to normal. United Airlines canceled 9% - or 266 - of Sunday's flights, the second most among carriers.

Delta CEO Ed Bastian said the issue affected its Microsoft Windows systems, snarling a critical application.

"In particular one of our crew tracking-related tools was affected and unable to effectively process the unprecedented number of changes triggered by the system shutdown," Bastian told customers in an email.

In a separate note, he told employees that Delta would continue to "tactically adjust" schedules to ensure safety.

Transportation Secretary Pete Buttigieg spoke with Bastian, according to an official, reminding him of the carrier's responsibilities to customers and the department's enforcement role.

"I will ensure that our department supports Delta passengers by enforcing all applicable passenger protections," Buttigieg said in a statement to Reuters, adding that the department had received hundreds of complaints.

"No one should be stranded at an airport overnight or stuck on hold for hours."

CrowdStrike said a significant number of the 8.5 million affected Microsoft devices were back online.